The invention is primarily directed towards a sensor for determining the moisture/complex dielectric constant and/or salinity of the medium in which the sensor is placed. To assist in the description of the invention we will use the example of one of its many uses, that of measuring the water and salinity content of soil but it will be understood that this is only an example and is not intended to be limiting in any way upon the scope of the invention as later claimed herein.
The efficient use of irrigated water for food production should be a primary objective of irrigators where the basic resources i.e. water, fertilisers, etc., are finite, degradable and/or costly. Efficient irrigation involves applying known amounts of water at frequencies which achieve optimum crop yield and quality, in a way which sustains and protects the resources being used. If irrigations are not correctly scheduled, food quality can be reduced, yields suppressed, perched water tables can rise, the degradation of irrigable land hastened and resources wasted. (The technique used to achieve this is referred to as "irrigation scheduling").
Proper irrigation scheduling requires decisions to be made regarding when to irrigate, how much water to use, and where it should be applied. Throughout the world, the irrigation industry recognises that there is a need for the development of an irrigation scheduling system which is inexpensive, accurate, reliable and repeatable. Gathering relevant information as part of this system is currently a very labour intensive and expensive undertaking, due partly to the instrumentation available and partly to the complexity of the situation.
There are many factors which influence irrigation scheduling techniques and the decision making process, comprising the climatic setting (arid, semi-arid etc.), soil texture, spatial variability, water supply (constraints on availability, cost of pumping, water quality), crop (flowering habit, harvest index, stress sensitivity at each stage of growth), irrigation system (degree of control, level of automation), weather (current, short term expected), economics (profit maximising level of irrigation) and last but not least the level of salinity in the soil water solution. The relationship between these factors can be highly complex, and the development of an integrated expert modelling system for practical irrigation scheduling and crop production has therefore been limited.
Much of the aforesaid problem has been due to the difficulty of collecting accurate and extensive data from the field. The development of a successful irrigation scheduling system based on monitoring will depend on the quantity and quality of the information collected. Some of the information related to the needs of a system is readily available and capable of being collated and understood by the agricultural decision maker without the need for specialised equipment.
This includes inventory information about the soil, water supply quantity and quality, the irrigation system, the physiology of the crop and environmental variables such as climatic details.
Of major significance to the success of an irrigation scheduling system is that part of the system which measures the response of the soil to the applied water and the rate of its depletion by the crop grown. Several devices and procedures have in the past been used to obtain soil moisture/complex dielectric constant measurements from which predictive models have been used to assist the irrigation decision making process. The most commonly used equipment and methods of collecting this type of data from the soil are outlined below.
The simplest but most labour intensive method for directly determining soil water content is the gravimetric sampling method. This methodology is the standard by which all other methods are calibrated. However, its high labour content makes it prohibitively expensive and is thus unsuitable for continuous monitoring of soil water conditions. It is also a destructive sampling technique which makes it impossible to return to the same location to determine the quantitative effects of "irrigation scheduling".
Gypsum blocks are simple and inexpensive devices for collecting moisture data in the field, and found favor in the past for these reasons. However, their main disadvantages lie in the uncertainty of calibration, and the short life expectancy of the block. In use the device is placed in the ground from which it absorbs moisture until equilibrium is reached. Then its resistance is measured from which the water content is inferred. However, this relationship varies between blocks and from soil to soil with time, due to changes in the conductivity of the soil solution in which the device is placed. Since moisture characteristics of the block are known to vary according to soil texture, the resistance/water content relationship derived for the block different soils also varies. However, taking into account the variable response of the block with time invalidates the earlier calibration.
Tensiometers are commonly used to schedule irrigations. They are relatively inexpensive instruments and operate by measuring the force with which water is held in the soil. Their major disadvantage is that the upper limit of measuring water potential is -0.8 bar. A further disadvantage is the need for frequent servicing to remove accumulated gases which are forced out of solution by the vacuum induced in the device during its operation.
Finally, the Bordon gauge versions of the tensiometer, which are the most common in use may have low accuracy and poor repeatability in the field.
The neutron probe is now a widely used instrument for measuring moisture content in the field. The probe generates fast neutrons from an internal radioactive source which are emitted into the soil through a cased hole into which the probe has been lowered during the measurement phase. Some of the emitted neutrons are reflected back after colliding with water molecules in the soil, and read by a detector in the base of the probe. A relationship exists between the number of neutrons detected and the water content of the surrounding medium, and by using a calibration equation, soil water content may be determined.
A serious disadvantage of a neutron probe is the required handling of a radioactive source with its attendant safety measures which not only restrict the general use of the device by the decision makers (farmers) but is also time consuming to operate. This device is further disadvantageous because the interpretation of raw data collected from the device further requires skilled analysis and customized computer assistance even before that data can be used in an "irrigation scheduling" system. The device has further disadvantages since it is not normally able to be left in situ over the periods of time required for data collection purposes for the dual reason of radiation element security and the expense of the device.
None of the abovementioned equipment and soil instrument methods permit frequent and efficient collection of data which is required for real-time assessment of the dynamic changes in water storage and movement in the soil being examined. A major reason for the lack of real-time measurement practices to allow objective scheduling by irrigators, apart from cost and labour, has been the inability of most of the systems available to immediately monitor dynamic soil water movement.